According to their scores on achievement tests, which correlate closely with IQ, GED recipients were every bit as smart as high-school graduates. But when Heckman looked at their path through higher education, he discovered that GED recipients weren’t anything like high-school graduates. At age twenty-two, Heckman found, just 3 percent of GED recipients were enrolled in a four-year university or had completed some kind of post-secondary degree, compared to 46 percent of high-school graduates. In fact, Heckman discovered that when you consider all kinds of important future outcomes – annual income, unemployment rate, divorce rate, use of illegal drugs – GED recipients look exactly like high-school dropouts, despite the fact that they have earned this supposedly valuable extra credential, and despite the fact that they are, on average, considerably more intelligent than high-school dropouts.

From a policy point of view, this was a useful finding, if a depressing one: In the long run, it seemed, as a way to improve your life, the GED was essentially worthless. If anything, it might be having a negative overall effect by inducing young people to drop out of high school. But for Heckman, the results also posed a confounding intellectual puzzle. Like most economists, Heckman had believed that cognitive ability was the single most reliable determinant of how a person’s life would turn out. Now he had discovered a group – GED holders – whose good test scores didn’t seem to have any positive effect on their lives.

What was missing from the equation, Heckman concluded, were the psychological traits that allowed the high-school graduates to make it through school. Those traits – an inclination to persist at a boring and often unrewarding task; the ability to delay gratification; the tendency to follow through on a plan – also turned out to be valuable in college, in the workplace, and in life generally. As Heckman explained in one paper: “Inadvertently, the GED has become a test that separates bright but nonpersistent and undisciplined dropouts from other dropouts.” GED holders, he wrote, “are ‘wise guys’ who lack the ability to think ahead, persist in tasks, or to adapt to their environments.”